Resistant pigweed: "Reduce seed bank"

University of Georgia Weed Scientist Stanley Culpepper stood at the edge of a field leaning against a hoe to help drive home a point — cotton producers should do whatever it takes to rid their crops of glyphosate-resistant Palmer amaranth pigweed, even if it means the drudgery of hand weeding.

During this year’s Sunbelt Ag Expo Field Day in Moultrie, Culpepper pointed out that a hoe could become the farmer’s tool of choice in the battle against cotton’s No. 1 weed pest. “This is surely the most problematic weed in all of cotton, but there are several very important things we have learned from our experience in dealing with it over the past four or five years,” he says.

A cotton plot at the Expo site has been in production for eight years, but for the first time this year, it has resistant Palmer amaranth, says Culpepper. “How did I get resistance? I didn’t bring in the seed. The wind blew the pollen from those resistant male plants — the male plant produces the pollen and the female plant produces the seed. The pollen came in last year and got into the female plants, and this year we have resistance. If we don’t do something about the female plant, it’ll produce half a million seed on dryland, pushing a million on irrigated.

“So if you don’t go out there and remove 10 or 20 plants on 19 acres, what happens next year? Multiply three-quarters of a million by 10, and then next year when you have a couple million out there, and they go to seed, what happens by year three? By year three, you cannot economically manage this pest. If you’re from south Georgia, you’re in years one or two in most of your fields, and you need to get it out of your fields. If you don’t catch it early, you’ll be behind the eight-ball in year three,” he says.

In addition to moving rapidly through pollen, resistant Palmer amaranth is also spread through traditional means such as custom harvesting, lack of cleaning equipment, and the spreading of invested materials.

Growers can’t afford to let this weed go to seed, says Culpepper. “You’ll have to change the things you are doing. I continue to see growers who are just spraying Roundup in Roundup Ready cotton. If you continue to do that, you will not survive. Even if you’ve survived this far, you will not survive in the future. You have got to use residual herbicides, and they have to get activated. If you have irrigation, you’re in great shape. If not, just spray and hope that’ll it help some,” he says.

Producers also might need to consider other strategies for battling glyphosate-resistant pigweed, he adds. “For example, there are Ignite-based programs, and a lot of growers are not even familiar with Ignite. You can’t apply it over Roundup Ready cotton — you have to have an Ignite-resistant variety. Those programs have proven to be very effective, especially in dryland production. You’re going to have to branch out and try some of these things. And of course, the plowing and the hand weeding have become much more common with the advent of glyphosate-resistant Palmer amaranth pigweed,” says Culpepper.

If growers don’t catch this weed pest early, it becomes more and more costly, he says. Some producers, says Culpepper, are spending as much as $100 per acre removing Palmer amaranth by hand. “It only gets worse if you don’t get aggressive with it. You need to be removing any pigweed from your fields now.”

Researchers, he says, continue to look at the benefit of using deep turning to control or prevent the spread of Palmer amaranth pigweed. “Last year, we knew we were losing the battle so we starting throwing a lot of packages together from many years ago. When we deep turned for the second year in a row, the plots looked identical. We’re reducing the seed bank that emerges in that crop by about 40 percent. So, if you had 100 plants emerging per square yard, you’d have 60 as a result of the deep turning. If you had 10 plants, now you have four. We’ve developed a program. It’s not good enough, but it might make things a little better.

“It clearly has been an effective tool, as has incorporating herbicides and Ignite-based programs. In conservation-tillage systems, heavy mulch has proven effective. Any one of these is not good enough, but the system seems to be coming together.”